The present invention relates to magnetic disk recording and more particularly to the recovery of data from double density floppy diskettes.
In the storage of data for computer and word processing applications, the desire to increase the amount of data which can be recorded on a given surface has led both to more sophisticated encoding techniques and to the use of high bit densities on the magnetic surfaces themselves. In storing large quantities of data on floppy diskettes, one of the increasingly popular methods is to utilize so-called double density recording, with encoding being done in the MFM mode. This is the recording technique utilized by International Business Machines (IBM) for its large capacity diskette systems. Since IBM is an industry leader and the original source of the floppy diskette recording technique, there exists a substantial advantage to employing this format.
In double density recording, however, the bit pattern on the inner tracks of the disk is actually so dense that magnetic interaction actually causes the bits to shift with respect to one another, i.e. as compared with the idealized time base employed during recording. As is well understood, a given nominal recording frequency will produce a higher bit density at the inner tracks, assuming a constant rate of rotation of the diskette is employed.
In order to overcome this tendency toward bit shifting, some disk drive interface manufacturers have employed a precompensation scheme which alters the timing during the recording of bit patterns, the alteration being in a manner which is intended to cancel out the effects of bit shifting. These precompensated recordings, however, do not conform to the IBM established standard.
Instead of pre-compensation, some disk drive manufacturers and interface manufacturers have chosen to provide some sort of post compensation or filtering to enable double density recorded data to be recaptured. While these latter techniques have met with some success, they have typically been limited in that they have been not allowed the use of all of the originally provided for track positions. In other words, the innermost tracks were unusable.
The most commonly utilized forms of filtering or post-compensation systems are based upon a phase locked loop and most floppy disk controllers provide for the use of such a system in regenerating data window timing. More recently, it has also been proposed to utilize a counter to generate a series of equal data windows following the receipt of a data bit from the floppy disk device. Again, these devices are limited in their ability to deal with bit shifting at the innermost tracks of a double density floppy disk recording, particularly when the recording is encoded in the MFM mode.
Among the several objects of the present invention may be noted the provision of apparatus for recovering data serially recorded on magnetic media at a high density tending to produce bit shifting; the provision of such apparatus which will read double density MFM encoded floppy diskettes; the provision of such apparatus which is highly reliable; and the provision of such apparatus which is of relatively simple and inexpensive construction. Other objects and features will be in part apparent and in part pointed out hereinafter.